1. Field of the Invention
The present invention relates to an absorbent for capturing carbon dioxide, the absorbent including an amino acid with multiple amine groups and a metal hydroxide, and more particularly, to an absorbent used in an absorption process that removes carbon dioxide included in flue gases emitted from a process emitting a vast quantity of greenhouse gases, for example, a coal-fired power plant, a steel mill, a petrochemical process, and a cement business.
2. Description of the Related Art
As global climate change is getting worse, various attempts to reduce greenhouse gases have been made. Many countries focus on reducing emission of greenhouse gases, while maintaining continuous development. Developing a renewable energy that is an alternative for current fossil fuels takes time, and the International Energy Agency (IEA) expects that fossil fuels will account for at least 70% of energy expected to be consumed in 2050. Therefore, emission of greenhouse gases is inevitable [IEA, Energy Technology Perspective (2006)].
According to a Blue MAP scenario released by the IEA, 19% of technologies for reducing carbon dioxide that makes up a greatest portion of the greenhouse gases will be technology for carbon dioxide capture and storage, in 2050.
The technologies for capturing carbon dioxide emitted from a large scale process may be classified as a pre-combustion technology, oxyfuel-combustion technology, and a post-combustion technology. The post-combustion technology is applicable to a conventional process, and is expected to be commercialized since the technology is readily applied.
Technologies for absorbing carbon dioxide may be classified as a chemical absorption process and a physical absorption process, and is widely used in the oil industry, the natural gas industry, and the chemical industry. An absorption capacity of a chemical absorption solvent for carbon dioxide is significantly higher than a physical absorption solvent at a low pressure, whereas the absorption capacity of the physical absorption solvent is higher than the physical absorption solvent at a high pressure.
An absorbent regeneration technology of the chemical absorption process may include a heating process to regenerate the absorbent, and an absorbent regeneration technology of the physical absorption process may decrease a pressure in a system. An economical efficiency in the absorption process depends on a technology of increasing the absorption capacity of carbon dioxide and reducing an amount of energy consumed for regeneration of the absorbent. An organic amine-based compound, for example, alkanolamine, may be mainly used in the chemical absorption process. The mainly used amines may be classified as a primary amine including monoethanolamine (MEA) and diglycolamine (DGA), a secondary amine including diethanolamine (DEA) and di-isopropylamine (DIPA), and a tertiary amine including triethanolamine (TEA) and methyl-diethanolamine (MDEA).
An absorbent having the highest absorption rate is MEA classified as the primary amine, and MEA absorbs carbon dioxide of 0.5 mol per absorbent of 1 mol. However, MEA consumes the highest amount of energy for regeneration of MEA, that is, 4.0 GJ/ton CO2, which is a major drawback of MEA. An absorbent showing the greatest efficiency among amine absorbents developed so far is the KS series, that is, a sterically hindered amine developed by Misubishi Heavy Industry (MHI) and the KANSAI electric power Co. in Japan. The sterically hindered amine may be hydrolyzed into bicarbonate due to instability of carbamate formed when the sterically hindered amine absorbs carbon dioxide and thus, carbon dioxide is absorbed by the absorbent, carbon dioxide of 1 mol per absorbent of 1 mol.
Even through an amine absorption process is evaluated as a process to be easily commercialized along with an ammonia absorption process, the amine absorption process includes the following drawbacks.
Cost of Resupplying Absorbent
When an alkanolamine absorbent absorbs carbon dioxide and is regenerated, a heat stable salt is formed by oxygen and heat, the loss of solvent and a predetermined amount of absorbent is resupplied due to a loss caused by evaporation (MEA 1.5 kg/ton CO2, KS-1 0.35 kg/ton CO2).
Corrosion of Device Due to Absorbent
When a concentration of alkanolamine is increased to increase an amount of absorbed carbon dioxide, a device may become corroded. Therefore, the concentration of alkanolamine may be decreased or a corrosion inhibitor may be added.
High Regenerative Heat Used for Regeneration of Absorbent
A high regenerative heat may be used to cut off a strong chemical combining between an absorbent and absorbed carbon dioxide during a regeneration reaction.
Loss of Absorbent and Side Reaction
When an absorbent is applied to flue gases including a sulfur component, a tiny amount of SO2 remaining after a flue gas desulfurization (FGD) device may cause a side reaction. A permissible level of SOx for KS-1 is less than or equal to 1.5 ppm. A reclaimer that is a separate device may be used to regenerate an absorbent degraded by the side reaction, and the reclaimer may consume additional energy.
One of processes to overcome the above described defects is a carbon dioxide absorption process using an amino acid salt. TNO in Holland filed an application associated with a process using an absorbent manufactured as the amino acid salt by mixing taurine amino acid and potassium hydroxide (KOH), and the application was granted (US publication No. 2006/0117954, international publication No. WO 03/095071, U.S. Pat. No. 7,591,878, and U.S. Pat. No. 5,749,941). When the absorbent used in the patent of TNO absorbs carbon dioxide, a salt is generated. Accordingly, when the absorbent is used in the absorption process, a membrane process or a spray tower may be used instead of a conventional absorption tower. Carbamate is generated as an intermediate according to an absorption reaction mechanism, and carbon dioxide of 0.5 mol per amino acid of 1 mol is absorbed.
The amino acid absorbent of TNO is less reactive to oxygen than an amine absorbent and thus, may not generate a heat stable salt, and a heat of absorption reaction is 10% less than MEA [Energy Proccedia 1, 1043-1050, 2009]. However, a higher absorption capacity for carbon dioxide and a higher carbon dioxide absorption rate may be needed to embody an economical process of capturing carbon dioxide.